Transmissible spongiform encephalopathy (TSE) or prion disease causes fatal neurodegenerative diseases in humans and other mammals. Of those, scrapie in sheep and BSE in cattle are particularly widely known. The disease in humans includes sporadic Creutzfeldt-Jakob disease (sCJD), iatrogenic Creutzfeldt-Jakob disease, Gerstmann-Straeussler-Scheinker (GSS) syndrome, fatal familial insomnia (FFI), and Kuru. The prion disease may occur by the conformational change of a natural normal prion to a variant abnormal prion (a variant prion protein that may cause the prion disease), which infects humans and other mammals. The variant prion protein has a β-sheet-rich structure compared with the normal prion protein, so the variant prion protein has high hydrophobicity, easily forms a multimer, and has resistance to protease K.
In recent years, some reports have revealed that variant CJD occurred mainly in the United Kingdom is caused by consumption of beef from a cow infected with BSE (Non-Patent Documents 1 and 2). There has been suggested that variant CJD may be transmitted by consumption of beef and transmitted from human to human by transfusion of a blood product or by transmitting a variant prion protein present in a graft of a tissue. Under the circumstances, in 2004, two cases were reported where recipients transfused with blood of a donor who developed variant CJD after blood donation were infected with the variant CJD, and in 2006, the third case was reported, so there is a very strong possibility that transmission of the variant CJD may occur by transfusion. There is quite a lot of humans who are infected with variant CJD but do not develop the disease, and blood products derived from such humans may spread the infection. Therefore, a method of removing an abnormal prion protein from a blood product is required.
In the field of a blood transfusion, a so-called leukocyte-removed blood transfusion has been spread, by which a blood product is transfused after the leukocyte contained in the blood product is removed. This is because it has been clarified that side effects such as headache, nausea, chill, and nonhemolytic febrile reaction which accompany the blood transfusion; grave side effects such as alloantigen sensitization, posttransfusion graft-versus-host disease (GVHD), and virus infection which seriously affect the recipient are caused mainly by the leukocyte mixed in the blood product used in the blood transfusion. A filter method has such advantages as excellent leukocyte-removing capability, simple operation, and low cost, so the filtering method has been widely used as a method of removing leukocytes from a blood product. In processing a blood product, blood has been generally filtered at the blood center, in recent years, before storage to carry out thorough quality control of a leukocyte-removed blood product by the filter method, that is, by using a leukocyte removal filter. In general, in the case where blood is filtered using a leukocyte removal filter at the blood center, a blood bag containing a blood product to be filtered is placed at a position higher than a recovery bag for the post-filtration blood product the by 70 cm to 150 cm, and the blood product is filtered based on action of gravity.
As sets for preparing a leukocyte-removed blood product, two sets (SCD-type and inline-type sets) are widely used. In the SCD-type set, a bag containing a blood product intended for removing leukocytes is aseptically connected to the set to remove leukocytes. Accordingly, only the leukocyte removal filter and the blood bag for recovering a blood product after filtration are connected. In the inline-type set, the process from blood recovery from a donor to preparation of a blood product is performed in an integrated system, so the blood bag generally contains a preservative solution or anticoagulant. To sterilize the SCD-type set, radiation sterilization is generally employed because of low cost. However, the radiation sterilization may cause decomposition of the preservative solution and anticoagulant, thus autoclave sterilization is generally employed for the inline-type set.
Degree of hemolysis is among the indices of the quality of a blood product containing erythrocytes. To supply a high-quality leukocyte-removed blood product containing erythrocytes, the Hemolysis Level should be less than 0.8% (Non-Patent Document 3).
From a viewpoint of operativity at the blood center, cost, and loss of a blood product, the method of removing an abnormal prion protein from a blood product is preferably a method of removing an abnormal prion protein and leukocyte simultaneously.
Patent Document 1 discloses a polymer for coating a leukocyte removal filter material including a unit originating from a hydrophobic polymerizable monomer, a unit originating from a polymerizable monomer containing a basic nitrogen-containing part, and a unit originating from a polymerizable monomer containing a protonic neutral hydrophilic part as a material having high leukocyte-removing capability, but does not disclose and suggest removal of an abnormal prion protein. Filtration of leukocyte-removed blood before storage includes, in many cases, both room-temperature filtration in which blood is filtered at room temperature within one day of collection of the blood and low-temperature filtration in which blood is filtered after blood storage in a refrigerator for about 1 to 3 days. In the case of the room-temperature filtration, more leukocytes leak than refrigerated blood although the filtration time is short, while in the case of the low-temperature filtration, the filtration time is long, and leakage of leukocytes may be relatively prevented. However, in Patent Document 1, no study has been made on the filtration time in the low-temperature filtration.
Patent Document 2 discloses a method of forming a complex of a prion protein in a biological fluid and a polymer matrix having a hydrophilic, hydrophobic, or amphiphilic functional group or a prion-binding substance including alumina or silica. However, the complex cannot remove leukocytes although the functional group is linked to a resin in examples, and it is necessary to use a leukocyte removal filter in actual use at the blood center. If removal of leukocytes and removal of abnormal prion proteins are performed separately, loss of a blood product, labor at the blood center, and cost may increase. In addition, alumina and silica are known to induce activation of a clotting system and show high nonselective adsorption for proteins, and hence the substances are not suitable for a blood product.
Patent Document 3 discloses a method of removing prions from an arbitrary liquid sample by using an apparatus such as a flow-through column and spherical polymer beads, the surfaces of which are coated with a prion complexing agent such as a metal salt (such as sodium) of phosphotungstic acid. However, it is necessary to expose the sample to the complexing agent for enough time to form complexes of the prion complexing agent and substantially all abnormal prion proteins in the sample. For example, the sample is incubated at about 30° C. to 45° C. (preferably 37° C.) for about 1 hour to 16 hours. However, the temperature of 37° C. is not suitable as a temperature for storing a blood product, and in a conventional method of using a leukocyte removal filter, room-temperature or low-temperature filtration is generally employed. In addition, in such method, filtration is performed based on action of gravity, so the method is not suitable for removal of abnormal prion proteins or leukocytes from a blood product.
Patent Document 4 discloses a method of forming a complex of a prion protein and a polymer matrix having a hydrophilic, hydrophobic, or amphiphilic functional group for removing or detecting a prion, or a prion-binding substance including alumina or silica. However, the prion-binding substance used in examples cannot remove leukocytes, and if removal of leukocytes and removal of abnormal prion proteins are performed separately, there are problems that loss of a blood product, labor at the blood center, and cost may increase. In addition, alumina and silica are known to induce activation of a clotting system and have high nonselective adsorbability for proteins, and hence the substances are not suitable for a blood product.
Therefore, in order to remove an abnormal prion protein from a blood product, a method of efficiently and easily removing the abnormal prion protein is required, and the method is further desired to remove the abnormal prion protein and leukocyte simultaneously.    [Non-Patent Document 1] G. Chazot, et al., (1996) Lancet 347:1181    [Non-Patent Document 2] R. G. Will, et al., (1996) Lancet 347:921-25    [Non-Patent Document 3] Guide to the preparation, use and quality assurance of blood components 9th edition/Council of Europe Publishing    [Patent Document 1] WO 03/011924    [Patent Document 2] US 2005/0014196 A    [Patent Document 3] JP 2002-539081 A    [Patent Document 4] JP 2006-522344 A